This invention generally relates to welding apparatus and methods and more specifically relates to an arc welding apparatus and method for precisely welding a joint or seam between abutting pipe portions that may reside in a confined space.
It is known that arc welding consists of fusing or joining at least two pieces of metal by establishing a metallurgical bond between them. A portion of the pieces to be joined by the welding process are subjected to intense heat for transforming the metal into a liquid state, which liquified metal is then allowed to solidify to produce a fused joint between the pieces. The heat, which may approximate 6500 degrees Fahrenheit, is generated by an electric arc which is a sustained electrical discharge generated between the workpieces and an electrode that is held sufficiently close to the metal workpieces to be welded. To create the electric arc, an electric welding power supply is connected to the electrode for supplying electric current to the electrode and a ground lead is connected to the workpiece for completing the electric circuit. Moreover, filler metal in the form of filler wire may be used to fill a groove or a predetermined gap that may be defined by the pieces to be welded. Such a groove or predetermined gap, which is formed by machining and/or properly positioning the pieces to be welded, is designed to prevent weld cracking and angular distortion in the completed weldment. Moreover, such a groove or gap may be necessary to achieve full penetration in welds that are more than 0.125 inch thick. The filler metal, which is held adjacent to the electrode, is fed into the molten weld puddle created by the electric arc and is added to the groove or gap during the welding process. The filler metal forms an intermolecular bond between the melted filler metal and the base metal of the pieces to be welded.
It is important that the arc length, and thus the distance of the tip of the electrode to the workpiece, be precisely maintained during the welding process. An arc that is overly long dissipates heat into the surrounding atmosphere and may cause molten metal to be scattered or spattered. Dissipation of the heat into the surrounding atmosphere may also result in a colder fluid weld puddle which may in turn cause imperfections in the weld joint. Thus, an arc voltage control unit is used in many mechanized applications to maintain a constant arc gap. Moreover, the speed of welding will be slower in the instance of an overly long arc because a longer time will be required to melt the required amount of metal due to some of the heat energy being dissipated into the surrounding atmosphere rather than being absorbed by the workpiece. In addition, a long arc is more susceptible to arc blow than a short arc. Arc blow is the deviation or wobble of the arc due to magnetic flux generated in the electrode by the current flowing in the electrode. The presence of arc blow will cause the arc to be erratic and unstable. All the factors recited hereinabove determine the quality of the completed weldment.
As stated hereinabove, filler metal may be used to fill a groove or a predetermined gap defined by the pieces to be welded. When filler metal is used, it is preferable that the filler metal be precisely positioned with respect to the electrode, the arc and the weld puddle for producing a quality weld in a reasonable time. It is important to provide and maintain a suitable wire entry angle with respect to the electrode, the arc and the weld puddle. A suitable filler wire entry angle is generally considered in the art to be between 15 degrees and 45 degrees with respect to the work surface plane which is the plane perpendicular to the electrode. Factors that determine the selection of a suitable filler wire entry angle include the diameter of a gas cup which surrounds the electrode, the extent of electrode projection from the gas cup, the weld joint geometry, the filler wire material and also whether the weld is made on the inside of a pipe, the outside of the pipe, or on a flat plate. In order to obtain a suitable wire entry angle, many prior art devices require virtually hundreds of brackets and attachments having different sizes and configurations. Therefore, these prior art devices are typically time-consuming to install and adjust and are complicated and cumbersome to use. Typically, after the arc establishes a fluid puddle of melted metal, the filler metal is slowly added to the weld puddle.
In addition to providing a suitable filler metal entry angle, positioning the filler wire relative to the electrode and the weld puddle is also important. The filler metal, electrode, and direction of travel of welding preferably should all be substantially in the same plane with minimal side angle. In addition, the filler metal should not enter the arc stream; rather, the filler metal should be precisely placed in the envelope of a suitable shielding gas which is caused to flow around the electrode and the arc stream. The shielding gas, which may be an inert gas (e.g., helium, argon) or an inert gas mixed with a reactive gas (e.g., oxygen, carbon dioxide), is used to expedite the welding process and to obtain a high-quality weld by decreasing defects such as hydrogen embrittlement, underbead cracking, and porosity. Thus, it is important that the filler metal be precisely positioned in relation to the shielding gas, the workpiece, and the electrode for producing a quality weld in a reasonable time. Therefore, a problem in the art is to precisely position the filler metal in relation to the shielding gas, the electrode, and the workpiece for producing a quality weld in a reasonable time.
Of course, arc welding can be used to weld abutting ends of stationary pipe sections. Moreover, when used to weld abutting ends of stationary pipe sections, such devices may use a rail assembly mounted around the pipe, upon which rail assembly an electrode assembly, which grips the electrode, is in turn mounted. The electrode assembly, which holds the electrode, may then be moved on the rail around the pipe for producing the desired shape and quality of weld. Thus, it is important that the rail assembly be securely connected to the pipe so that the electrode assembly precisely moves around the rail for producing a quality weldment in a reasonable time. Therefore, another problem in the art is to precisely secure the rail assembly to the pipe and to precisely secure the electrode assembly to the rail assembly such that the electrode assembly will move on the rail around the pipe in a precise and predetermined manner for producing a quality weldment in a reasonable time.
Moreover, it may be required to perform the welding of the pipe portions in a confined space where the clearance for welding is limited. That is, the pipe portions to be welded may reside close to another pipe structure, an adjacent wall structure, nearby equipment, or other structures. Therefore, in such applications, it is preferable that the welding apparatus be configured to obtain a minimal volumetric envelope so that the required welding can be performed where clearances for welding are limited. Consequently, another problem in the art is to provide an apparatus for welding pipe sections where the clearance for welding is limited.
Devices for welding pipe sections are known. One such device is disclosed by U.S. Pat. No. 4,649,250 issued Mar. 10, 1987, in the name of Gasparas Kazlauskas entitled "Pipe Welder". The Kazlauskas patent discloses a pipe welder having a welding head housing. A welding electrode housing, upon which is mounted a welding electrode, is slidably mounted onto the welding head housing. The Kazlauskas device includes a hollow tube for guiding filler wire to the weld site. The aft end of the hollow tube is pivotally mounted onto a bracket and the bracket is mounted on a pin which is supported by an arm. The arm is pivotally mounted onto a different mounting bracket belonging to the pipe welder device. Filler wire is conducted through the hollow tube to a position directly adjacent the tip of the welding electrode.
A jig for butt welding or cladding pipes or shafts is disclosed by German Offenlegungsschirft 30-00-399 published June 19, 1981, in the name of Gebruder Sulzer AG. This device includes a welding burner that is mounted on a trolley driven in either direction around a workpiece and fitted with two wire feeders. According to this document, a circular rail made of hinged segments is detachably mounted on a round workpiece, and a trolley carrying a welding head is driven in either direction through 360 degrees around the rail.
Although the welding art discloses various devices for welding, the art does not appear to disclose an arc welding apparatus and method for precisely welding a joint or seam between abutting pipe portions that may reside in a confined space.
Therefore, there is clearly a need for an arc welding apparatus and method for precisely welding a joint or seam between abutting pipe portions which may reside in a confined space.